1. Field of the Invention
The present invention relates to an AD conversion device in an integrating system for AD converting the analog information of a camera.
2. Description of the Prior Art
A conventional integrating-type AD conversion circuit for AD converting the analog exposure information of a camera, AD converts an analog signal obtained from the light sensing element for measuring the object brightness. Another conventional system, AD converts analog signals other than those obtained from the light sensing element, for example, analog signals delivered from a flash unit to the camera.
Brightness under room illumination changes at a frequency of 100 Hz or 120 Hz, two times as often as the conventional frequency of 50 Hz or 60 Hz. It is thus necessary to select an integrating time of about 10 msec. for the AD conversion to avoid the influence of changes in brightness of the illumination when the analog signals is obtained from the light sensing element for measuring the object brightness. Thus, in the conventional AD converter for a camera, the integrating time for the AD conversion is selectively set at about 10 msec.
Further, in a conventional camera, the integrating time for the AD conversion of the analog signal transmitted from the flash unit to the camera, despite the absence of any effects of a commercial frequency, is also selectively set at about 10 msec. This is equal to the time for the AD conversion of the analog signal obtained from the light sensing element that measures the object brightness.
A conventional AD conversion circuit of a camera appears in FIG. 1.
FIG. 1 includes an operational amplifier 1, an SPD light sensing element 2 for measuring the object brightness, a diode 3 for logarithmically compressing the photoelectric current, an operational amplifier 4, resistors 8 and 9 for dividing the reference voltage in half, an operational amplifier 10 for buffering the voltage divided by the resistors, analog switches 12 to 15 brought into the conductive state when the control signals are at a high level. Switch 15 serves to reset the charge of a capacitor 17. A resistor 16 determines the integrating current. An integrating capacitor 17 and an operational amplifier 18 together form an integrator. Element 19 is a comparator. A counter latch circuit 20 counts the clock pulses from the start of the output of an RS Flip-Flop (hereinafter called RS-FF) 21 until the start of the output of the comparator 19 and latches the counter with the start of the output of the comparator 19. Element 21 is an RS-FF. A timer circuit 22 produces a pulse after the lapse of a predetermined time, for example, 10 msec. after the start of the ADSTART signal for starting the AD conversion. The circuit also includes an RS-FF 23, an inverter 24, AND gates 25 and 26, and a NOR gate 27.
In operation, the analog exposure information from the flash unit is applied to the one input of the analog switch 12 via the voltage follower of the operational amplifier 1. On the other hand, the photoelectric current corresponding to the object brightness produced by the light sensing element 2 is logarithmically compressed into the light measurement value by a logarithmic compression amplifier composed of the operational amplifier 4 and the diode 3. This compressed value is applied to the analog switch 13.
Further, the voltage Vc is divided in half by the resistors 8 and 9 and applied to the input of the analog switch 14 via the voltage follower of the operational amplifier 10. Before the start of the AD conversion, the ADSTART level is low, the RS-FF 23 is not set so that its output Q is low, while the RS-FF 21 has not been set before the start of the AD conversion, and the analog switch 15 is closed via the gate 27. Thus, the charge in the integrating condenser, composed of the condenser 17 and the operational amplifier 18, is reset and the output of the operational amplifier 18 is Vc.
When the AD conversion then starts and the signal ADSTART rises from an L level, i.e. low, to an H level, i.e. high, the timer circuit 22 starts the clocking or clock pulses, while the RS-FF is set to open the analog switch 15 via the NOR gate 27. When the AE/EF signal for showing whether the flash unit mounted on the camera has been charged or not is high, i.e. when the camera is not yet in the flash mode, the output of the AND gate 26 is high and the analog switch 13 is closed. Consequently, the current proportional to the difference between the voltage corresponding to the object brightness produced by the operational amplifier 4 and the reference voltage Vc flows from the output of the operational amplifier 4 to the integrating capacitor 17 via the analog switch 13 and the resistor 16 so that the integrating capacitor 17 charges and the output voltage of the operational amplifier 18 lowers in proportion to the amount of charge. When the output OUT of the timer circuit 22 is inverted from low to high 10 msec. after the ADSTART changes from low to high, the RS-FF 23 is reset and the output goes low, while the RS-FF 21 is set and its output Q goes high. Thus, the closed analog switch 13 opens and the opened analog switch 14 is closed, while the counter latch circuit 20 starts the counting. Thus, the charge in the integrating capacitor 17 is discharged to the output of the operational amplifier 10 via the resistor 16 and the analog switch 14 in such a manner that when the charge in the integrating capacitor become 0, i.e. when the output of the operational amplifier 18 reaches the reference voltage Vc, the comparator 19 is inverted and the counter latch circuit 20 stops the counting of the clock and latches the clock. The output of the counter latch circuit 20 corresponds to the output of the operational amplifier 4, namely the signal corresponding to the object brightness, which is AD converted.
Until now, the integrating time when the camera is in the flash mode and the output voltage of the operational amplifier, which is the signal from the flash unit, is AD converted is determined by the timer circuit 22 and therefore equal to the integrating time for AD conversion of the analog signal.
On the other hand, a time lag exists between the time a shutterbutton reaches its second step, i.e. its shutter release step, until the actual start of the shutter. This is because the quick mirror in the camera is raised, the diaphragm of the photographic lens is closed by the camera and, for the automatic exposure control, the output of the light sensing element for measuring the object brightness is AD converted to be calculated with other photographic information. This delay creates a danger of missing a shutter chance, i.e. an opportunity to take a picture. It has thus been an important problem to decrease the time lag. A hindrance to decreasing the above-mentioned time lag is that the integrating time of the integrator in the AD conversion circuit for AD converting the analog signal from the flash unit is equal to that of the integrator in the AD conversion circuit for AD converting the output of the light sensing element for measuring the object brightness.